1. Field of the Invention
This invention relates to a band brake assembly and more particularly, to a band brake assembly for use in a vehicle and having a drum with improved lubrication flow control.
2. Background of the Invention
Band brakes are generally employed within automatic transmissions in order to perform gear shifts within the transmissions. For example and without limitation, shifting a xe2x80x9cstep-ratioxe2x80x9d type automatic transmission is accomplished by selectively applying and releasing friction elements such as band brakes which are assigned to various rotating elements or gears (e.g., planetary gears) within the transmission.
A conventional band brake includes a band, a drum, and a hydraulic servo. The band-drum interface is lubricated with an oil. The drum is directly connected to a planetary gear or other automotive transmission component. When the band brake is actuated, it frictionally engages the drum, effective to hold connected components while altering a gear configuration for automatic shifting. Thus, the band engagement dynamics directly affect automotive transmission shift quality. Consistent band behavior in repeated engagements or under varying operating conditions is vital to providing and maintaining good shift quality.
However, in a conventional automotive transmission system, the band-drum engagement behavior varies significantly due to varying lubrication flow into the band-drum interface. Typically, the lubricant (e.g., oil) is first routed to the interior surface of the drum from the drum hub shaft. The rotation of the drum exerts a centrifugal force on the oil layer inside the drum. The amplitude of this force depends on the rotational speed of the drum and the oil layer thickness. The drum has small lubrication holes drilled through its shell. The centrifugal force pushes the oil through those holes into the band-drum interface. Prior art has relied upon the size, number, and location of lubrication holes to regulate the oil flow into the interface. However, the oil layer thickness inside the drum still varies widely under different operating conditions. As a result, a large variability still exists in the actual lubrication flow rate, causing widely varying engagement behavior. The band-drum engagement variability makes automatic transmission shift calibration more difficult, leading to inconsistent shift quality.
Referring now to FIG. 1, there is shown a sectional view of a prior art band brake assembly 10. As shown, a conventional brake band assembly 10 typically includes a band 12, a drum 14, and a hub shaft 16 which receives engine generated torque which is distributed through the transmission. The drum 14 is fixedly attached to the hub shaft 16 or other rotating components (not shown). The drum 14 shown in FIG. 1 rotates with the hub shaft 16. The drum is generally hollow and typically houses other conventional transmission components 26. The drum includes several oil-lubrication holes 18 which are typically drilled through the drum 14. The hub shaft 16 includes oil distribution channels 20 which are formed within the shaft 16 and which communicate with apertures 22 which release and/or discharge the oil into the inside of drum 14. When a relatively large oil flow exists under certain transmission operating conditions, a relatively thick oil layer 24 may accumulate inside the drum 14. This relatively thick oil layer 24 may induce an excessive lubrication at the band-drum interface unless the oil is drained elsewhere. This excessive lubrication is particularly detrimental to the engagement behavior at low oil temperatures due to strong hydrodynamic effects. It may also lead to increased parasitic torque loss when the band is not engaged. While some attempts have been made to direct the oil into a low flow resistance area for draining purposes, this has led to lubrication deficiencies at the band-drum interface at high oil temperatures. These lubrication deficiencies potentially cause a very abrupt engagement behavior, increased band-drum interface temperature, and reduced band durability.
There is therefore a need for a new and improved band brake drum assembly which overcomes many, if not all, of the previously delineated drawbacks of such prior vehicle band brake drum assemblies, and which provides for reduced engagement variability.
A first non-limiting advantage of the invention is that it provides a band brake drum assembly which overcomes at least some of the previously delineated drawbacks of prior assemblies and which effectively controls the oil layer thickness inside the drum under all operating conditions, thereby providing a consistent oil flow into the band-drum interface and consistent engagement behavior.
A second non-limiting advantage of the invention is that it provides a band brake drum assembly having lubrication flow control attributes which are effective to reduce the variability in lubrication flow within the band interface.
A third non-limiting advantage of the invention is that it provides a band brake drum assembly for use in an automatic transmission system which reduces the variability in engagement and shifting characteristics.
A fourth non-limiting advantage of the invention is that it provides a band brake drum assembly having reduced parasitic torque loss at the band-drum interface due to regulated lubrication flow.
A fifth non-limiting advantage of the invention is that it provides a band brake drum assembly with improved oil draining flow which is effective to reduce parasitic torque loss in the rotating components of the assembly.
According to a first aspect of the present invention, a drum is provided for use within a band brake assembly. The drum includes interior surface which receives lubricant, a shoulder which is disposed upon the interior surface and which forms a lubricant retention area for holding an amount of the lubricant, at least one lubricant hole formed within the lubricant retention area, the hole communicating with a band-drum interface exterior to the drum and allowing at least a portion of the lubricant to be communicated from the drum to the band-drum interface, and at least one lubricant draining hole which is formed outside of the lubricant retention area and which is effective to drain excess lubricant from the drum.
According to a second aspect of the present invention, a band brake assembly is provided and includes a selectively actuatable brake band member; a selectively rotatable shaft having at least one channel which selectively distributes an amount of lubricant and an aperture which communicates with the channel and which selectively discharges the lubricant; and a generally cylindrical rotating drum which is fixedly coupled to the selectively rotatable components such as a shaft, the drum including an interior surface which receives an amount of lubricant, a recessed lubricant retention portion which is formed upon the interior surface and which selectively receives and holds an amount of the discharged lubricant, a plurality of lubricant holes formed within the lubricant retention portion, the lubricant holes communicating with an exterior portion of the drum which is selectively engaged by the brake band member and allowing at least a portion of the lubricant to be communicated to the exterior portion of the drum, and a plurality of lubricant draining holes which are formed outside of the lubricant retention portion and which is effective to drain excess lubricant from the drum.
According to a third aspect of the present invention, a method is provided for controlling the amount of lubricant within a transmission band brake drum. The method includes the steps of: forming a lubricant retention area on the inside surface of the transmission band brake drum, effective to retain a certain amount of lubricant; and draining excess lubricant from the band brake drum which flows outside of the lubricant retention area.
These and other features, aspects, and advantages of the invention will become apparent by reading the following specification and by reference to the following drawings.